1. Field of the Invention
The present invention relates to an assembly type crankshaft, and more particularly to the assembly type crankshaft to be assembled by pressing a crank pin of a crankshaft and into a crank web of the crankshaft.
2. Description of Background Art
In a conventional assembly type crankshaft to be assembled by pressing a hollow crank pin into a pair of crankshafts (crankshaft webs) held to oppose to each other, the hollow crank pin has been incorporated in advance with a part such as a connecting rod. The above-described both crankshafts (crankshaft webs) are moved to be closer to each other to thereby press the hollow crank pin comparatively lightly into a pin hole in the crankshaft. Thereafter, a cylindrical column-shaped piece member (plug member) having a slightly larger diameter than an inner diameter of a hollow hole of the hollow crank pin is caused to be pressed in from both sides of the hollow crank pin (See, for example, Japanese Patent Laid-Open No. 60-37411 (1st page and 2nd page, FIG. 2)
The above-described conventional assembly type crankshaft described in Japanese Patent Laid-Open No. 60-37411 has a structure in which as shown in FIG. 6. Between a pair of crankshafts 02, 02 (crankshaft webs) held to oppose to each other, there is arranged a hollow crank pin 01 incorporated with a part such as a connecting rod 04 in advance. The above-described both crankshafts 02, 02 are moved to be closer to each other to thereby press both ends of the hollow crank pin 01 comparatively lightly into pin holes 02a, 02a in the crankshafts 02, 02. Then, after the pressing-in has been completed, a cylindrical column-shaped piece member 03, 03 having a slightly larger diameter than an inner diameter of a hollow hole 01a of the crank pin 01 is pressed in from both sides of the hollow crank pin 01.
In the conventional assembly type crankshaft to be assembled by the above-described press-in, the pressing of the hollow crank pin into a pin hole in the crankshaft has been performed by means of comparatively light fitting. As such, a press-in allowance has been made to become relatively small, whereby in an assembly stage due to the press-in, the arrangement has been made in such a manner that a deformation of the crankshaft due to the assembly is confined to a minimum. After the assembly, the piece member (plug member) is pressed into the hollow hole in the hollow crank pin from its both ends, whereby the outer diameter of the hollow crank pin is bulged out to impart required surface pressure to the inner peripheral surface of the pin hole in the crankshaft, resulting in a crankshaft having a firm fixed state.
According to the above-described assembly structure, since substantial fixing of the assembly type crankshaft due to pressing-in depends on surface pressure to an inner peripheral surface of a pin hole in the crankshaft to be imparted by the piece member (plug member) which is pressed into the hollow portion of the hollow crank pin, it is necessary that the piece member (plug member) has a predetermined length in order to impart the sufficient surface pressure.
In large-sized types of engines, however, the rigidity of their crank pins is generally high, and in such a state, pressing of the piece member (plug member) into the hollow hole of the hollow crank pin in the assembly, that is, pressing of the long piece member into the hollow hole becomes considerably difficult because of excessive press-in load. Thus, since the excessive press-in load brings about press-in deformation of the assembly type crankshaft, a process of correcting the deformation is generally required, thus deteriorating the productivity.
Further in a four-stroke cycle engine, since the pin hole in the crankshaft is to be used as a passage of lubricating oil, the use of the long piece member causes a problem in setting the passage for the lubricating oil.
Also, the use of the long piece member increases the weight of the crank pin, and the increase in the weight of the crank pin increases the counterweight, resulting in an increase in stress in the engine during the engine driving, and an increase in vibrations during high-speed rotation of the engine.
Thus, appropriate measures for solving the above-described problems in the conventional assembly type crankshaft are desired, but in fact, until the development of the present invention, a good plan had not been found.